Microlens is widely employed in a variety of fields, such as micro-electro-mechanical systems (MEMS), image sensors including charged coupling device (CCD) image sensors, complimentary metal-oxide-semiconductor (CMOS) image sensors, photoelectric and photonic devices and so on.
In general, a microlens is utilized to guide the light to the photosensitive component, and typically acts as a focusing element. A microlens is a tiny lens formed on a semiconductor substrate above a photosensitive component. Because the light passes through the microlens, it is important that the microlens is shaped accurately for guiding the light on the photosensitive component. In some examples, the microlens transforms a light pattern into an electric charge pattern.
Microlens is generally formed by a hard molding method. However, the hard molding method is difficult for fabricating small microlens patterns and keeping the same uniformity across a large area.
Another method involves a photo-patternable polymer and a thermal reflow process. Microlens is often formed by patterning a polymer layer formed over a color filter or a photosensitive component, a dielectric layer, or other substrate features. A thermal reflow process is subsequently performed to heat the patterned polymer for a deformation which creates a desired shape of the microlens thereby. In some example, each microlens is aligned over the corresponding image sensors formed in the underlying substrate. Consequently, the incident light on each microlens is focused towards the corresponding image sensors. In such process, the curvature of microlens is fixed and limited by the property of the polymer and the required process temperature for the reflow step is high.
Nevertheless, a concerned issue incurred during the fabrication of the microlens as the design sizes are reduced. For example, as the design size is reduced, the microlens is positioned closer together. As the microlens is positioned closer together, the microlens has a tendency to merge during the thermal reflow procedure. The reflow procedure employed in high temperatures is difficult to control and also difficult to prevent the microlens from merging as the distance between microlens shortens.
Another concerned issue is related to the focal length. The thickness of the device will increase due to the additional layers as more circuitry is integrated onto a semiconductor chip. Because the additional layers increase the focal length, a thinner microlens is required consequently. The thermal reflow process for creating thinner microlens generally uses a much higher temperature and it leads to increasing possibilities of the microlens merging.
There is another concerned issue in optical interconnections and optical waveguides. The fabricating processes of an optical waveguide are facing problems similar to those for fabricating a microlens. In some example, an extra microlens is required to be configured to utilize the optical waveguide.
There is a need to solve the above deficiencies/issues.